1. Field of the Invention.
This invention relates to acid reprocessing and more particularly, to a double distillation reprocessing of spent piranha acid to obtain semiconductor grade sulfuric acid (H.sub.2 SO.sub.4).
2. Prior Art.
Various methods for reprocessing acid are known, with double distillation being reasonably well known. Double distillation may be used when reprocessing spent piranha acid which is a combination of H.sub.2 SO.sub.4 and an oxidant, such as H.sub.2 O.sub.2. In the semiconductor integrated circuit manufacturing industry this combination (piranha acid) is used to clean wafers and strip the photoresists.
The corrosive and toxic nature of the cleaning and stripping acids (e.g., H.sub.2 SO.sub.4) presents several problems. First, the problem of disposal is an economic problem as well as an environmental problem. Economically, it is costly to properly dispose of spent acid. Strict environmental control must be maintained. Even when environmental regulations are observed and strictly complied with, there is always the possibility of environmental pollution. Spilled acid travels quickly through the ground layer resulting in contamination of the aquifer and eventually reaches the ground water. In addition to the downward movement, a spill can creep through the ground sideways, thus creating a growing contamination. In addition to the damaged environment, the liability cost associated with a hazardous waste clean-up is high, thus resulting in an even greater economic loss. Second, the piranha acid can only be used once and when spent must be discarded, thereby requiring purchase of additional acid. This is costly and restarts the disposal cycle.
Alternatively, piranha acid can be reprocessed, thus breaking this economically and environmentally costly cycle. While acid reprocessing appears clearly to be a viable alternative, any acceptable process must produce H.sub.2 SO.sub.4 of sufficient purity to exceed the semiconductor industry standards for sulfuric acid. The industry requires H.sub.2 SO.sub.4 to be extremely pure, with total metallic impurities being less than 500 ppb, typically much less. Particulate matter resulting from the cleansing of wafers and stripping of photoresist must also be minimized. Precise understanding of the source of particles in the liquid is as yet unrealized. However, small amounts of particulates can significantly reduce the yield of semiconductor chips. For example, during the photolithographic process, small particulates can adhere to the wafer and result in the loss of a transistor or a conductor line, thereby resulting in a low yield, i.e. an increased loss of functional chips per wafer. Contaminates are even more disastrous when the process is used for VLSI fabrication.
It is believed that the high particulate count probably partially results from the necessity of using high boiling temperatures in a second stage of a double distillation process. Double distillation is a process used to purify acids and has been an established technology for approximately 30 years. In the first distillation, low boiling compounds, such as water, carbon dioxide and unreduced compounds are distilled off from the acid. The acid, having a higher boiling point, remains in the distillation mixture and is transferred to the second distillation stage. The distillation mixture transferred to the second stage contains the higher boiling point acid and other high boiling point compounds, such as heavy metal contamination and particulate.
Theoretically, these contaminants remain in the distillation vessel after the high purity acid is distilled off. However, in practice, a small portion of these contaminants and particulates are distilled off. The particulates escape the liquid phase of the mixture and are carried over into the distillation column in the prior art second stage distillation along with the gaseous H.sub.2 SO.sub.4.
Accordingly, considerable effort has been directed to devising a process that reduces this carry over improving the purity of H.sub.2 SO.sub.4. In particular, much effort has been directed at evolving a process that decreases the particulate matter and other contaminants.